1. Field of Invention
The present invention relates to measurement of chromatic dispersion characteristics of a DUT (Device Under Test) such as an optical fiber, and in particular to a technique for measuring dispersion characteristics without being influenced by the expansion and contraction of the DUT.
2. Description of the Related Art
When measuring chromatic dispersion characteristics of a device-under-test (DUT) such as an optical fiber and the like, it is desired that the measurement can be performed while excluding the influence of expansion and contraction of the DUT. A technique for measuring the DUT without being influenced by the expansion and contraction thereof is disclosed, for example, in Japanese Patent Laid-Open No. Hei 1-291141.
The construction of a measuring system is shown in FIG. 4. As shown in FIG. 4, the measuring system is divided into a light source system 10 and a characteristic measuring system 20. A variable-wavelength light source 12 of light source system 10 changes the wavelength, so that a light of wavelength xcexx (variable-wavelength light) is generated. A fixed-wavelength light source 13 fixes the wavelength, so that a light of wavelength xcex0 (fixed-wavelength light) is generated. And, xcex0 is the wavelength at which the chromatic dispersion is minimized in an optical fiber 30. The variable-wavelength light and fixed-wavelength light are modulated by an optical modulator 15a and an optical modulator 15b, respectively, to a frequency f, and multiplexed in an optical multiplexer 16.
The light multiplexed in the optical multiplexer 16 is entered into the optical fiber 30. The light transmitted through the optical fiber 30 is entered into an optical demultiplexer 21 of the characteristic measuring system 20. The optical demultiplexer 21 divides the light transmitted through the optical fiber 30 into a light of wavelength xcexx and a light of wavelength xcex0. A photoelectric converter for measurement 22a and a photoelectric converter for reference 22b perform photoelectric conversion of the light of wavelength xcexx and the light of wavelength xcex0, respectively, and a phase comparator 24 detects the phase difference between outputs of the photoelectric converter for measurement 22a and the photoelectric converter for reference 22b. 
The transmitted light of wavelength xcexx is influenced by the chromatic dispersion, and the expansion and contraction of optical fiber 30. The transmitted light of wavelength xcex0 is influenced only by the expansion and contraction of optical fiber 30. This is because xcex0 is the wavelength at which the chromatic dispersion is minimized in the optical fiber 30. Therefore, if the phase difference between the transmitted light of wavelength xcexx and the transmitted light of wavelength xcex0 is detected, it is possible to exclude the influence caused by the expansion and contraction of optical fiber 30.
However, in order to allow the optical demultiplexer 21 to divide the light transmitted through the optical fiber 30 into the light of wavelength xcexx and the light of wavelength xcex0, the wavelength xcexx and the wavelength xcex0 should be somewhat separated. It is difficult for wavelength bands of wavelength xcexx and wavelength xcex0 to be common. For example, the wavelength xcexx is from 1525 to 1635 nm and the wavelength xcex0 is 1300 nm, so that the wavelength xcexx and the wavelength xcex0 should be somewhat separated.
Therefore, the object of the present invention is to provide an apparatus that can perform measurement of chromatic dispersion, even if the wavelength of variable-wavelength light source and that of fixed-wavelength light source for reference are identical with each other.
According to the present invention described in claim 1, an apparatus for measuring optical characteristics of a device-under-test which transmits a light, includes: a variable-wavelength light source for generating a variable-wavelength light, the wavelength of which is variable; a fixed-wavelength light source for generating a fixed-wavelength light, the wavelength of which is fixed; a first optical modulation unit for casting an incident light for measurement on one end of the device-under-test, wherein the incident light for measurement is the variable-wavelength light subjected to intensity modulation to a predetermined frequency; a second optical modulation unit for casting an incident light for reference to the other end of the device-under-test, wherein the incident light for reference is the fixed-wavelength light subjected to intensity modulation to a predetermined frequency; a transmitted light obtaining unit for measurement that obtains a transmitted light for measurement, which is the incident light for measurement transmitted through the device-under-test; and a transmitted light obtaining unit for reference that obtains a transmitted light for reference, which is the incident light for reference transmitted through the device-under-test, wherein the apparatus measures the characteristics of device-under-test based on the transmitted light for measurement and the transmitted light for reference.
According to the optical characteristic measuring apparatus constructed as explained in the above, the variable-wavelength light is transmitted from the one end to the other end of the device-under-test, whereas the fixed-wavelength light is transmitted from the other end to the one end of the device-under-test. Therefore, it is possible to separate and obtain the variable-wavelength light and fixed-wavelength light transmitted through the device-under-test regardless of their wavelengths. Therefore, it is possible to measure the chromatic dispersion even if the wavelength of variable-wavelength light source is identical with the fixed-wavelength light source for reference.
According to the present invention described in claim 2, an apparatus for measuring optical characteristics of a device-under-test which transmits a light, includes: a transmitted light obtaining unit for measurement that obtains a transmitted light for measurement which is an incident light for measurement, transmitted through the device-under-test, wherein the incident light for measurement is a variable-wavelength light, the wavelength of which is variable, subjected to intensity modulation to a predetermined frequency and then cast on one end of the device-under-test; a transmitted light obtaining unit for reference that obtains a transmitted light for reference which is an incident light for reference, transmitted through the device-under-test, wherein the incident light for reference is a fixed-wavelength light, the wavelength of which is fixed, subjected to intensity modulation to a predetermined frequency and then cast on the other end of the device-under-test; a photoelectric conversion unit for measurement that performs photoelectric conversion of the transmitted light for measurement obtained by the transmitted light obtaining unit for measurement; a photoelectric conversion unit for reference that performs photoelectric conversion of the transmitted light for reference obtained by the transmitted light obtaining unit for reference; a phase comparison unit for detecting a phase difference between the phase of output of the photoelectric conversion unit for measurement and the photoelectric conversion unit for reference; and a characteristic calculation unit for calculating group delay characteristics or chromatic dispersion characteristics of the device-under-test using the phase difference.
The present invention described in claim 3, is an apparatus for measuring optical characteristics as claimed in claim 2, wherein the transmitted light obtaining unit for measurement includes a first terminal for measurement into which a light is entered, a second terminal for measurement from which the light entered into the first terminal for measurement exits and into which a light is also entered, and a third terminal for measurement from which the light entered into the second terminal for measurement exits, wherein the transmitted light obtaining unit for reference includes a first terminal for reference into which a light is entered, a second terminal for reference from which the light entered into the first terminal for reference exits and into which a light is also entered, and third terminal for reference from which the light entered into the second terminal for reference exits, and wherein the incident light for reference is entered into the first terminal for measurement, the other end of the device-under-test is connected to the second terminal for measurement, the incident light for measurement is entered into the first terminal for reference, and the one end of the device-under-test is connected to the second terminal for reference.
The present invention described in claim 4, is an apparatus for measuring optical characteristics as claimed in claim 3, wherein the transmitted light obtaining unit for measurement and the transmitted light obtaining unit for reference are directional couplers.
The present invention described in claim 5, is an apparatus for measuring optical characteristics as claimed in claim 3, wherein the apparatus further includes: a photoelectric conversion unit for measurement that performs photoelectric conversion of the output of the third terminal for measurement, a photoelectric conversion unit for reference that performs photoelectric conversion of the output of the third terminal for reference, a phase comparison unit for detecting a phase difference between the phases of outputs of the photoelectric conversion unit for measurement and the photoelectric conversion unit for reference, and a characteristic calculation unit for calculating group delay characteristics or chromatic dispersion characteristics of the device-under-test using the phase difference.
According to the present invention described in claim 6, a method for measuring optical characteristics of a device-under-test which transmits a light, includes: a variable-wavelength light generating step for generating a variable-wavelength light, the wavelength of which is variable; a fixed-wavelength light generating step for generating a fixed-wavelength light, the wavelength of which is fixed; a first optical modulation step for casting an incident light for measurement on one end of the device-under-test, wherein the incident light for measurement is the variable-wavelength light subjected to intensity modulation to a predetermined frequency; a second optical modulation step for casting an incident light for reference to the other end of the device-under-test, wherein the incident light for reference is the fixed-wavelength light subjected to intensity modulation to a predetermined frequency; a transmitted light obtaining step for measurement that obtains a transmitted light for measurement, which is the incident light for measurement transmitted through the device-under-test; and a transmitted light obtaining step for reference that obtains a transmitted light for reference, which is the incident light for reference transmitted through the device-under-test, wherein the apparatus measures the characteristics of deviceunder-test based on the transmitted light for measurement and the transmitted light for reference.
The present invention described in claim 7, is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristic measuring process for measuring a characteristic of a device-under-test which transmits a light, the optical characteristic measuring process including: a variable-wavelength light generating processing for generating a variable-wavelength light, the wavelength of which is variable; a fixed-wavelength light generating processing for generating a fixed-wavelength light, the wavelength of which is fixed; a first optical modulation processing for casting an incident light for measurement on one end of the device-under-test, wherein the incident light for measurement is the variable-wavelength light subjected to intensity modulation to a predetermined frequency; a second optical modulation processing for casting an incident light for reference to the other end of the device-under-test, wherein the incident light for reference is the fixed-wavelength light subjected to intensity modulation to a predetermined frequency; a transmitted light obtaining processing for measurement that obtains a transmitted light for measurement, which is the incident light for measurement transmitted through the device-under-test; and a transmitted light obtaining processing for reference that obtains a transmitted light for reference, which is the incident light for reference transmitted through the device-under-test, wherein the apparatus measures the characteristics of device-under-test based on the transmitted light for measurement and the transmitted light for reference.
According to the present invention described in claim 8, a method for measuring optical characteristics of a device-under-test which transmits a light, includes: a transmitted light obtaining step for measurement that obtains a transmitted light for measurement which is an incident light for measurement, transmitted through the device-under-test, wherein the incident light for measurement is a variable-wavelength light, the wavelength of which is variable, subjected to intensity modulation to a predetermined frequency and then cast on one end of the device-under-test; a transmitted light obtaining step for reference that obtains a transmitted light for reference which is an incident light for reference, transmitted through the device-under-test, wherein the incident light for reference is a fixed-wavelength light, the wavelength of which is fixed, subjected to intensity modulation to a predetermined frequency and then cast on the other end of the device-under-test; a photoelectric conversion step for measurement that performs photoelectric conversion of the transmitted light for measurement obtained by the transmitted light obtaining step for measurement; a photoelectric conversion step for reference that performs photoelectric conversion of the transmitted light for reference obtained by the transmitted light obtaining step for reference; a phase comparison step for detecting a phase difference between the phase of output of the photoelectric conversion step for measurement and the photoelectric conversion step for reference; and a characteristic calculation step for calculating group delay characteristics or chromatic dispersion characteristics of the device-under-test using the phase difference.
The present invention described in claim 9, is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristic measuring process for measuring a characteristic of a device-under-test which transmits a light, the optical characteristic measuring process including: a transmitted light obtaining processing for measurement that obtains a transmitted light for measurement which is an incident light for measurement, transmitted through the device-under-test, wherein the incident light for measurement is a variable-wavelength light, the wavelength of which is variable, subjected to intensity modulation to a predetermined frequency and then cast on one end of the device-under-test; a transmitted light obtaining processing for reference that obtains a transmitted light for reference which is an incident light for reference, transmitted through the device-under-test, wherein the incident light for reference is a fixed-wavelength light, the wavelength of which is fixed, subjected to intensity modulation to a predetermined frequency and then cast on the other end of the device-under-test; a photoelectric conversion processing for measurement that performs photoelectric conversion of the transmitted light for measurement obtained by the transmitted light obtaining processing for measurement; a photoelectric conversion processing for reference that performs photoelectric conversion of the transmitted light for reference obtained by the transmitted light obtaining processing for reference; a phase comparison processing for detecting a phase difference between the phase of output of the photoelectric conversion processing for measurement and the photoelectric conversion processing for reference; and a characteristic calculation processing for calculating group delay characteristics or chromatic dispersion characteristics of the device-under-test using the phase difference.